1. Field of the Invention
The present invention relates to a compact zoom lens system for a video camera in which defocus of the lens system is automatically detected in the camera and the focusing operation of the lens system is automatically controlled by the camera.
2. Description of the Prior Art
Recently, since the compactness of a plurality of electric circuits contained in the video camera system including a lens system is improved, the lens system comprises a large proportion of the size and the weight of the whole video camera system. Also, the cost of the lens system represents a significant percentage of the cost in the whole video camera system. Therefore, a lens system for use in a video camera system is required to be compact in size, light in weight and low in cost.
Conventionally, there is known a four component zoom lens system for a video camera comprising from the object side, a first positive focusing lens group, a second negative variator lens group, a third negative compensator lens group, and a fourth positive master lens group, for example, by Japanese Laid-Open Patent Application No. 61-93423. In the zoom lens system of this type, the first positive focusing lens group is shiftable along the optical axis in the focusing operation. Therefore, the diameter of the first positive focusing lens group is very large, and possess over 60% in weight and size of the whole lens system. Thus, it is difficult for making a lens system more compact and lighter, if a zoom lens system of this type is applied.
Here, in order to decrease the diameter of the first positive focusing lens group, the zoom lens system of a plurality of other types has been proposed in which a part of the variator lens group, the compensator lens group, the master lens group, a part of the compensator lens group, or a part of the master lens group is shiftable in the focusing operation. However, in the zoom lens systems of the types described above, the shifting distance of the focusing unit shiftable in the focusing operation is changed in accordance with the change of the focal length of the lens system due to the zooming operation. Therefore, the shifting mechanism becomes complex in construction. However, in recent video camera systems the shifting distance of the focusing unit can be automatically controlled by an automatic focusing unit provided in the video camera system by using a comparatively simple construction.
Another approach for making the zoom lens system more compact is to shift a plurality of lens groups in the zooming operation. However, the construction of such a focusing mechanism becomes more complex and increases the cost, and furthermore, such an approach has the following problems.
If the focusing lens unit is divided by two components which includes a first component located on the object side of an aperture diaphragm and a second component located on the image side thereof, an interlocking mechanism for interlocking the first lens component with the second component becomes very large, since an automatic aperture control mechanism provided for automatically controlling the aperture diaphragm is very large compared with the diameter of the lens groups. Therefore, the diameter of the whole lens system including the lens barrels becomes undesirably large. As a result, it is difficult to make the zoom lens system more compact when applying the above type of zoom lens system.
Furthermore, a zoom lens system of a type in which the first positive focusing lens group is shiftable also in the zooming operation, requires an additional interlocking mechanism for interlocking the first positive focusing lens group with the other focusing unit, in comparison with the zoom lens system in which the first positive focusing lens group is fixed in the zooming operation. Since it causes an increase in the size of the lens barrels, it is difficult to making the zoom lens system more compact.